1. Field of the Invention
The present invention relates to a lens meter for measuring the optical characteristics of a lens including spherical refractive power, cylindrical refractive power, axial angle and prism quantity and, more particularly, to a lens meter for measuring the additive diopter of a lens.
2. Description of the Prior Art
A known lens meter, such as "Auto Lens Meter LM-870" proposed by the applicant of the present patent application, determines the optical characteristics of a lens including spherical refractive power, cylindrical refractive power, axial angle and prism quantity on the basis of results of measurement of the locus of measuring light transmitted through the lens by a photo-detecting device. This known automated lens meter is capable of operating in an additive diopter measuring mode and of measuring the additive diopter of a progressive lens. When measuring the additive diopter of a lens by this known lens meter, the lens meter is set for the additive diopter measuring mode, the far viewing section of the lens is measured, the measured data of the far viewing section is stored in a memory, the lens is shifted to position the near viewing section of the lens at the measuring position, the data of the near viewing section is measured when the operator decides that the near viewing section is positioned at the measuring position, and the difference between the measured data of the far viewing section and that of the near viewing section is calculated to determine the additive diopter.
The respective positions of the far viewing section and the near viewing section in the lens are determined subjectively by the operator and the correctness of the determination is dependent on the perception and experience of the operator. Generally, a lens as manufactured is provided with marks indicating the respective positions of the far viewing section and the near viewing section. Therefore, the respective positions of the far viewing section and near viewing section of the lens as manufactured can be fairly accurately determined for measuring the characteristics of the lens. However, these marks are easily erasable, the marks are erased when the framed lens is wiped for cleaning and concealed marks are difficult to recognize visually. Therefore, the accurate measurement of the lens requires a skillful expert operator. Furthermore, any objective data securing correct measurement is not available.